While examining the production of various cytokines by human tumor cell lines, we have discovered that 6 of 6 melanoma cell lines tested constitutively produced M-CSF. In 5 of the cell lines tested, this production was relatively low (1-5 ng/ml when the cells were confluent) while 1 of the melanoma cell lines, RPMI 5971, released approximately 20- 100x more M-CSF. In several of the cell lines, the addition of PMA was found to increase M-CSF production 10-fold. In addition, we have preliminary evidence that M-CSF, when present in high enough concentrations, may act to inhibit the proliferation of these cells. Work is currently in progress to characterize the production of M-CSF by these melanoma cell lines and to determine if this cytokine plays a role in regulating the growth of these cells.